[WingsToDiscovery]

Unfortunately, it's all about supply and demand, and German isn't seen as much of a commodity compared to English. Your student probably sees it as

A) You're not a native English speaker (which, not matter how good you are, may have some restrictions in Japan)
B) Your Japanese isn't fluent either, so anything that may need serious explanation may not be able to be conveyed properly in Japanese to make the most sense.

She probably sees you as a convenient settlement, because she can still learn some basic English, but for a possible discount.

The good news is, you can always find new clients. As you've already stated, if she only thinks an hour a week is going to do anything significant, then it's time to find someone else.